Dihydropyrimidine dehydrogenase (DPD) is the first and rate limiting enzyme in the three step metabolic pathway of the catabolism of thymine and uracil. Among the end products of pyrimidine catabolism is beta- alanine. By virtue of its ability to degrade pyrimidines, DPD is responsible for the metabolism of fluoropyrimidine drugs, such as the extensively used chemotherapeutic agent 5-fluorouracil (5-FU). A number of patients with cancer receiving a therapeutically adjusted dose of 5-FU exhibited unexpected severe toxicity that, among a variety of clinical symptoms, included neutropenia, thrombopenia and neurological damage. A number of cases of 5-FU toxicity have been documented that are related to low DPD activities. The association of high levels of toxicity with a simple decrease in enzyme activity is not particularly surprising since most anticancer drugs exhibit narrow therapeutic indices. Marked inter-patient variability with respect to pharmacokinetics for most chemotherapeutic drugs used in cancer treatment can result in adverse toxicity and lack of efficacy. Thus, careful determination of the patient's metabolic capacity to inactivate a specific anticancer agent, such as 5-fluorouracil, can minimize adverse side-effects and maximize efficacy, resulting in a better prognosis for each individual patient. The molecular basis for DPD deficiency in a British family having a cancer patient that exhibited grade IV toxicity 10 drug after 5-FU treatment was analyzed. A 165-bp deletion spanning a complete exon of thee gene was found in some members of the pedigree having low DPD catalytic activity. Direct sequencing of lymphocyte DNA from these subjects revealed the presence of a G to A point mutation at the 5'-splicing site consensus sequence (GT to AT) that leads to skipping of the entire exon preceding the mutation during pre-RNA transcription and processing. A PCR-based diagnostic method was developed to determine that the mutation is found in caucasian and asian populations. This mutation was also in a Dutch patient with thymine-uraciluria and completely lacking DPD activity. Many of these patients exhibit severe disabilities including abnormal mental development, convulsions and altered physical features such as microcephaly. A genotyping test for the G to A splicing point mutation could be useful in predicting cancer patients prone to toxicity upon administration of potentially toxic 5-FU and for genetic screening of heterozygous carries and homozygous deficient subjects.